Belt tensioner and belt drive

ABSTRACT

A tensioner with a pulley rotatably mounted to a pivot arm that is biased with a spring means, the pivot arm includes a leg extension for inhibiting movement of the arm with a damping means.

This is a division of application Ser. No. 07/588,633 filed Sep. 26,1990 now U.S. Pat. No. 5,098,347.

BACKGROUND OF THE INVENTION

The invention relates to a belt tensioner, and a belt tensioning system,but more particularly, the invention relates to a spring type tensionerthat may be used as part of a synchronous belt drive system. While thetensioner of the invention may be used in different belt drive systemapplications, it is particularly useful in solving belt tension problemsassociated with camshaft belt drive systems for automotive applications.

Synchronous belt systems, like most mechanical systems, includecomponents with inertias and elasticities and, as a result, such beltsystems are capable of vibration and can be characterized from avibration point of view, by natural frequencies. The most important typeof system vibration that relates to the invention is angular vibrationwhere angular vibration is for example, the oscillation of pulleys abouttheir axes of rotation superimposed over relatively constant angularvelocities of those pulleys of the system.

Vibrations can be detrimental to the system operation. The worstcondition may occur at system resonance when the forces associated withpulley motion (i.e., angular vibration) oscillate with a frequency equalto the system natural frequency.

In a camshaft drive system, a synchronous belt is entrained aroundpulleys that include at least two toothed pulleys. One of the pulleys isa crank pulley and the other is one or more camshaft pulleys that inducecyclic torque variations into the drive system. An idler pulley such asa back side idler pulley is pressed against the belt to effect aninstalled belt tension.

The one or more camshafts of an automotive belt drive operate over arange of frequencies and induce cyclic torque variations into the drivesystem. At the frequency of torque variation equal to the system naturalfrequencies the resonance will occur. Such resonance frequencies willvary with different engine designs. The torque variations contribute toexciting forces that introduce maximum amplitudes of tension variationin the belt at the resonance frequencies. The amplitudes of high tensionoccur as the camshaft pulleys move dynamically in an opposite directionor at a slower or opposite angular rate than the crankshaft pulley. Whenthe amplitudes of vibration are too high, belt failure may occur by theteeth being sheared off of the synchronous belt at the crank pulley.

It is common practice in many synchronous belt drive systems to minimizethe amplitude of maximum resonance tension variations in the systemspans by means of a fixed idler pulley. An installation belt tension ata room temperature is chosen to inhibit the amplitude of the variationsto prevent belt tooth failure and avoid tooth jump (ie., ratcheting). Ifthe tension is too low, tooth failure and tooth jump can occur. Ifinstallation tension is too high, it can introduce shortened belt lifeand belt noise at engine operating temperatures. The fixed idler pulleymust operate over a range of temperature conditions. There is a changein pulley center distance between a cold engine such as usually occursduring engine start up and a warm or thermally expanded engine such asoccurs during normal engine operating temperatures. Thus, the thermaleffect is to increase belt tension with engine temperature increases andconversely decrease belt tension with decreases in engine temperature.

The advantage of a fixed idler is that it operates without substantiallyeffecting stiffness of the belt drive system to minimize resonancetension variations due to dynamic effects as introduced by torquevariations in the drive. The disadvantage of a fixed idler system isthat it is often difficult to consistently set installation belttensions at a desirable level. System natural frequency and amplitude ofvibration at the resonance depends on the belt installation tension. Ifthe tension is too low, resonance will occur which can contribute todynamic belt failure. If the tensions are too high, noise will resultalong with belt failure due to over tensioning. A more detailedexplanation of the above discussed vibration associated with dynamiccharacteristics of an automotive camshaft drive system appear in SAETechnical Paper Series No. 880077 by Mizuno, et al.

It might appear to the unskilled that belt tensioners for automotivevehicle acessory such as for example those shown in U.S. Pat. No.4,299,584 should provide a solution to overcome the variationsattributable to thermal and dynamic effects associated with anautomotive synchronous drive. Such tensioners are unsatisfactory asbeing "too soft" because they easily move in an attempt to compensatefor belt tension changes as associated with tension variationsintroduced by cyclic torque variations of the cams. Such accessory drivetensioners adjust to variations in belt tensions by moving an idlerpulley against a belt. The movements of the idler pulley are damped toinhibit the amplitude of tensioner vibrations. The damping torque isusually less than 35 percent of the damping force that is effected bythe tensioner in maintaining a drive tension. Consequently, the trendfor tensioners that accommodate both dynamic and thermal conditions inan automotive camshaft drive are of the hydraulic type such as, forexample, shown in U.S. Pat. No. 4,883,446. While such hydraulictensioners overcome the problem of a fixed idler as is associated withthermal expansion, installation tension, and over tensioning that maycause noise, they also introduce their own problems. Such tensioners areusually very expensive, and they depend on an oil system forpressurizing the actuator which system may fail causing either an overtensioning or under tensioning the belt.

SUMMARY OF THE INVENTION

In accordance with the invention, a tensioner is provided that is usefulin conjunction with belt drive systems and is particularly useful in asynchronous belt drive system to compensate or handle tension variationsattributable to dynamic effects such as those introduced by cyclictorque variations at a pulley in the drive system, or thermal effectssuch as those that introduce a change in the length of a synchronousbelt drive. The invention also relates to a method of tensioning asynchronous belt drive system so as to compensate for thermal anddynamic effects on belt tension as well as belt stretch and wear.

The belt tensioner of the invention is of the pivot type with an idlerpulley rotatably mounted to a pivot arm, a spring to bias movement ofthe arm, and a damping means with a leg extension of the pivot arm forinhibiting movement of the arm where the damping force is sufficient toinhibit arm movements under some dynamic conditions. When used in asynchronous belt drive system, the tensioner has a spring means forbiasing the moveable pulley against the belt with a force to limit theamplitude of vibration at resonance that may be introduced into the beltsuch as by cyclic torque variations. The damping in such a belt drivesystem is sufficient for inhibiting movement of the pulley with thedamping force that is greater than the dynamic exciting force at thetensioner.

An object of the invention is to provide a spring biased tensioner thatis suitable for use in a belt drive system and, particularly, asynchronous belt drive system where there are thermal and dynamic belttensioning effects.

Another object of the invention is to provide an inexpensive spring typebelt tensioner that is capable of handling thermal and dynamic tensioneffects as well as belt stretch and wear effects in a synchronous beltdrive system and which also provides suitable stiffness for the drive toinhibit effects attributable to high tension amplitudes associated withthe resonance of the drive system.

These and other objects or advantages of the invention will be apparentafter reviewing the drawings and description thereof wherein:

FIG. 1 is a schematic front view of the synchronous belt drive system ofthe automotive type and which includes a belt tensioner of theinvention;

FIG. 2 is a view taken generally along the line 2--2 of FIG. 1 showingan enlarged partially broken away view of a belt tensioner of theinvention;

FIG. 3 is a view taken along the line 3--3 of FIG. 2; and

FIG. 4 is a view taken along the broken line 4--4 of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

While the various features of the tensioner, belt drive system, andmethod of the invention are perphaps best described with a synchronousbelt drive for an automotive engine, many features of the invention mayalso be used in other belt tensioning applications such as automotivefrontend accessory drives. Referring to FIG. 1, a synchronous belt drivesystem 10 is shown with a toothed belt 12 entrained and tensioned aroundtoothed pulleys. The illustrated drive system is a camshaft drive of theautomotive type that includes two toothed cam pulleys 14, 16, a crankpulley 20, a tensioning pulley 22, a toothed water pump pulley 24 and astationery idler pulley 26.

As an automotive drive system, the pulleys operate over a range offrequencies following the operating RPM of the automotive engine and thecamshaft pulleys introduce cyclic torque variations into the drive.While some cyclic torque variations may be introduced at the crankpulley due to piston strokes of the engine, they are substantially minorin comparison to those introduced by the camshaft pulleys. However,torque variations introduced into the belt are reflected at the driverpulley where working conditions for belt teeth are most difficult due tothe small diameter of the crankshaft pulley and small number of teeth inmesh. The variations in torque contribute to an exciting forces thatintroduce amplitudes of tension variations over the range of operatingfrequencies in spans 28, 29 of the belt. As an automotive belt drivesystem, there are thermal growth changes of the engine that have aneffect on belt tension which are in addition to those attributable todynamic tension variations. A tensioner 30 of the invention compensatesfor thermal and dynamic belt tension changes while maintaining a "stiff"drive system as is later explained in greater detail in conjunction withthe operation of the tensioner.

Referring to FIGS. 2-4, a belt tensioner 30 of the invention is of thespring biased type and includes a pivot support structure 32, an armstructure 34 and a spring 36 biased between the support structure andarm structure. The arm structure 34 is pivotably mounted to the supportstructure by means of a pivot 38 and an optional self-lubricatingpolymeric sleeve-type bearing 40 that includes a thrust flange 42.

The pulley 22 is rotatably mounted to the arm such as by means of aroller bearing 44 and is operative at a moment arm M3 from the pivot asthe pulley moves with the arm structure in pressing engagement againstthe belt.

The spring is preferably in the form of a compression spring and isinterpositioned between a post 46 of the support structure and thespring is operative at a moment arm M1 in relation to the pivot 38.

A damping means 48 is included for inhibiting movement of the arm andhence the pulley against the belt when in use. The damping meansincludes a leg extension 50 of the arm structure, a damping spring 52attached 54 to the support structure, a surface 56 of the supportstructure and a pad 58 of friction material. The leg 50 as an extensionof the arm structure, pivotably moves therewith and defines an arcuatedamping zone as bounded by dotted lines 60, 62 as illustrated in FIG. 2.The damping spring may be a compression spring or is preferably in theform of a U-shaped leaf spring with a leg portion 64 juxtaposed thedamping zone.

While the pad 58 of friction material could be attached to the surface56 of the support structure or the leg 64 of the spring 52, it ispreferably carried by the leg 50. It is preferred that the leg have anaperture 66 and that the pad of friction material be disposed in theaperture and protrude at its opposite ends from oppositely facing sidesof the leg portion. In such an arrangement, the opposite ends of the padare in friction surface sliding contact with the face 56 of the supportstructure and leg 64 of the spring. An advantage of positioning thefriction material in the aperture of the leg is that it defines a meansto adjust to a zero clearance between the arcuate movement of the legportion and the surface of the support structure. The pad of frictionmaterial may be of any chosen type but it optionally may be in the formof a polymeric material such as that sold under the trademark Delrinwhich exhibits a starting (static) friction that is less than itssliding (dynamic) friction.

The leaf spring 52 in contact with the polymeric pad and surfaceprovides substantially a constant damping force at a moment arm M2 inrelation to the pivot 38. Preferably, the moment arm M2 for the dampingmeans is greater than the operative moment arm M1 for the spring meansto minimize the damping spring force while simultaneously preciselycontrolling the damping torque of the tensioner. Optionally, the momentarm M2 for the damping means is also greater than the moment arm M3 forthe pulley 22.

As illustrated by the vertical alignment of FIGS. 3 and 4 of the supportstructure, the spring 36, the damping means 48, the rotational mount ofthe pulley 22 and the pivot 38 are in substantially planer alignment forthe advantage of minimizing or eliminating offset moments that couldoccur if such elements were not in alignment. Such an arrangement hasthe advantage of minimizing bearing sizes, pivot sizes, spring sizes,and the like.

METHOD

To install a belt with the tensioner of the invention and at a precisebelt tension, the belt is entrained around all of the pulleys except forthe moveable pulley of the tensioner. A fastener 68 such as a bolt orpin is loosely fitted through an aperture 70 of the tensioner as thepulley 22 is loosely positioned against the back side of the belt. Alever means such as a wrench is inserted into a wrenching means such assquare shaped aperture 72 and torque is applied to the support structurewhich in turn presses the pulley 22 against the belt and slightlycompresses the spring. Another fastener such as a bolt 74 is insertedthrough another aperture 76 of support of the structure which ispreferably elongated for easy fastener alignment. The spring 36 alsooperates to rotate the support structure about fasteners 68 and pressthe aperture 76 against the fastener 74 so that the precise installationbelt tension is achieved.

As previously briefly mentioned, the selection of the biasing force totension the belt and the damping force to inhibit movements of thetensioner are of primary importance to provide a "stiff" drive systemand thereby minimize the amplitudes of system vibration. A tensionerthat does not provide the requisite force and damping would result in a"soft" system that would allow substantial tensioner pulley movementwhich could lead to catastrophic tension variations that would destroythe belt.

In tensioning a synchronous belt drive system that operates over a rangeof frequencies and where there are cyclic torque variations induced intothe drive system by one of the toothed pulleys, such torque variationscontribute to exciting forces in the belt that introduce angularvibration. The angular vibration is controlled by the method of biasinga moveable idler pulley against the belt with a force sufficient tolimit the amplitudes of the tension variations at the system resonance.The biasing force is also sufficient to induce an operative tension forthe belt drive system. The idler pulley axis movement must be reduced topractically zero so that it is not a source for introducing resonancetension variations in the belt. This is done by the process of dampingthe reciprocal movement of the idler pulley into the belt and brakingmost of the reciprocal movement of the pulley from the exciting forcesinduced into the belt by these cyclic torque variations. However, thedamping process simultaneously permits accommodating thermal growthchanges in the drive system by permitting change of the position of theidler pulley. It has been determined that a substantial amount ofdamping is required over that of the spring type accessory drivetensioners of the prior art. The damping force should be in a range fromabout 35 percent to less than 100 percent of the force reacted by theidler pulley from the belt. More preferably, the damping force is fromabout 40 percent to about 60 percent of the reactive force at the idlerpulley. With such damping, there is sufficient resistance to brakesubstantially all cyclic movements of the idler pulley while alsopermitting small change of the position of the idler pulley when thebelt tension exceeds the spring and damping force reflected at thepulley. By applying a damping force to the idler pulley, whose value isjust greater than the difference between the dynamic force applied tothe idler pulley by mean belt tension and that applied by the maximumdeviation from the mean tension, the tensioner remains substantiallyrigidly locked in position and will act as a fixed idler. An incrementalshift in mean belt tension will result in the increment tension plus themaximum deviation in tension exceeding the opposing spring andfrictional force, and this will cause the tensioner to move or "creep"until the belt force is again no greater than the spring and frictionalforce whereupon the "creep" stops. The result is good tension controlwithout an undesirable softening of the system.

To illustrate the effectiveness of the tensioner, synchronous belt drivesystem, and method, a tensioner of the invention was installed as areplacement for a fixed idler pulley of a toothed belt camshaft drive ofan automotive engine where the engine was required to operate betweenabout 750 rpm and 5500 rpm.

In the prior art fixed idler pulley system, the initial belt tension fora cold engine, ie., room temperature 70° F. was set for 40 lbs tension;under warm engine temperatures, ie., approximately 200° F., the nominalbelt tension increased to approximately 80 lbs. During engine warm up,belt tension variations of 153 lbs. were observed at the belt tensionerat an engine speed of 2500 rpm. The variation reduced to 74 lbs. at 2500rpm at engine operating temperature.

When a tensioner of the invention was installed in drive to provide acold tension of 80 lbs. and where the damping force was about 40 percentof the tensioning force, variations in tension remained at approximately74 lbs. while the engine reached its operating conditions. Only minormovements of the pivot arm were observed as the tensioner adjusted itsarm position for changing thermal conditions and tension in the belt.

The foregoing detailed description is made for the purpose ofillustration only and is not intended to limit the scope of the appendedclaims.

What is claimed is:
 1. In a synchronous belt drive system of the typewith a toothed belt entrained and tensioned around pulleys including atleast two toothed pulleys that operate over a range of frequencies wherethere are cyclic torque variations induced into the drive system by atleast one of the toothed pulleys and which torque variations contributeto exciting forces that introduce high amplitudes of tension variationsat system resonance within the operating range of frequencies in spansof the belt and wherein the improvement comprises:one of the pulleysmoveable against the belt; spring means for biasing the moveable pulleyagainst the belt with a force sufficient to limit the amplitude of theresonance tension variation in the span while inducing an operativetension in the belt; and a damping means for inhibiting movement of themoveable pulley against the belt to effect a damping force into the spanwhich is substantially equal to or greater than the exciting force atthe moveable pulley.
 2. The belt drive system as claimed in claim 1wherein the moveable pulley is an idler pulley rotatably mounted on anarm that is pivotably mounted by means of a pivot, a support structure,the spring means includes a compression spring that is biased againstthe pivot arm at a first moment arm from the pivot, and the dampingmeans includes a friction material attached to the pivot arm at a secondmoment arm from the pivot and pressed in frictional contact against thesupport structure.
 3. The belt drive system as claimed in claim 2wherein the moment arm for the friction material is greater than themoment arm for the spring.
 4. The belt drive system as claimed in claim2 wherein the belt drive system is an automotive engine drive systemwhere one of the toothed pulleys is a camshaft pulley which induces thecyclic torque variations and the other toothed pulley is a crank pulley.5. The belt drive system as claimed in claim 2 wherein the damping forceis substantially constant.
 6. The belt drive system as claimed in claim1 wherein the spring means in combination with the damping means andbelt define a means for 1) substantially rigidifying the moveable pulleyagainst movement during operation of the drive system over the range ofoperating frequencies and 2) allowing movement of the moveable pulley toaccommodate thermal expansions and contractions of the drive system andsubstantially inhibit belt tension changes attributable to thermalconditions, belt sheet and belt wear.
 7. A belt tensioner for tensioninga toothed belt of a synchronous belt drive system of the type with thebelt entrained and tensioned around pulleys including at least twotoothed pulleys that operate over a range of frequencies where there arecyclic torque variations induced into the drive system by at least oneof the toothed pulleys and which torque variations contribute toexciting forces that introduce high amplitudes of variations at systemresonance within the operating range of frequencies in spans of thebelt, the tensioner comprising:a pivot support structure; an armstructure pivotally mounted by means of a pivot to the supportstructure; an idler pulley rotatably mounted to the arm structure andengageable with the belt as part of the drive system; a spring biasedbetween the support structure and arm and operative at a moment arm fromthe pivot to press the idler pulley against the belt and with a forcesufficient to limit the amplitude of the resonate tension variation inthe span while inducing an operative tension in the belt; and a dampingmeans for inhibiting movement of the arm structure and thereby the idlerpulley against the span to effect a damping force into the span which issubstantially equal to or greater than the exciting force at the idlerpulley.
 8. The belt tensioner as claimed in claim 7 wherein the dampingmeans comprises 1) a pad of friction material attached to the armstructure at a moment arm from the pivot and, 2) a means for pressingthe pad in contact with the pivot support structure to effect a dampingforce.
 9. The belt tensioner as claimed in claim 8 wherein the momentarm for the pad is greater than the moment arm for the spring.
 10. Thebelt tensioner as claimed in claim 8 wherein the pressing means is aleaf-spring.
 11. The belt tensioner as claimed in claim 7 wherein thedamping force is sufficiently large to brake movements of the arm forsubstantially all exciting forces induced into the belt by the cyclictorque variations.
 12. A belt tensioner comprising:a means forreciprocally moving and biasing movement of an idler pulley to effect areactive force at the idler pulley; and a means for damping reciprocalmovement of the idler pulley to effect a damping force that is in arange from at least about 35 percent to less than 100 percent of thereactive force and where the damping force is sufficient to brakesubstantially all cyclic movements of the idler pulley while alsopermitting some movement in response to a force at the idler pulley thatis greater than the sum of the reactive force and damping force.
 13. Thebelt tensioner as claimed in claim 12 wherein the damping force is fromabout 40 percent to about 60 percent of the reactive force.
 14. The belttensioner as claimed in claim 12 wherein the reciprocal and biasingmeans includes a compression spring and pivotal structure that rotatablymounts the idler pulley.
 15. The belt tensioner as claimed in claim 12wherein the belt tensioner is a synchronous belt tensioner.